BRANCHIAL SENSE ORGANS IN ICHTHYOPSIDA. 117 
Vacus IN AMPHIBIA. 
Mr. Spencer has recorded in this Journal! certain observations 
on the nerves of Amphibians. He has found that not merely 
the ganglia of the dorsal roots of cranial nerves of Amphibians, 
but that the whole of the nerves themselves are split off from 
the skin. I have figured the origin of the vagus nerve and 
ganglion in the frog in fig. 27. I have investigated the facts 
in Amphibians, and can fully confirm Mr. Spencer in most 
points. The development as seen in Amphibians is interest- 
ing, as in some respects showing a very primitive condition of 
the nervous system, viz. a nerve sheath or part of one; in 
other respects it is impossible in them to get as good a view of 
the primitive nerve composition of the head as in Elasmo- 
branchs. 
In Amphibians a considerable amount of fusion of once 
separate nerves has taken place, not only behind the auditory 
organ, but also in front of it. As an instance, it may be men- 
tioned that the ciliary ganglion, which in Elasmobranchs, and 
even in birds, is quite distinct in its development, is in the 
Amphibians fused with the Gasserian, and the two arise 
together as one fused mass. 
Vagus 1, 2,3, and 4 are also all fused into one mass in 
Amphibia; the figure (27) is a transverse section through this 
mass. In it the nerve has not separated from the skin, and 
the ganglionic portion is readily recognisable as a mass of 
yolk-filled cells on the level of the lateral line. Later, both 
ganglion and nerve leave the skin as in Elasmobranchs. 
NERVE OF THE THIRD SEGMENT—TRIGEMINAL LESS 
OrutTHAL. PRoFuND. 
The fifth nerve is well suited for studying the development 
of the ganglion of a dorsal root. 
It is well known, from Balfour’s and Marshall’s researches 
(opera cit.), that it arises from the third of the brain vesicles. 
1 «Quart. Journ. Micr. Sci.,’ Supplement, July, 1885. 
